1. Field of the Invention
This invention relates to a lawn mower having selectively drivable wheels.
2. Description of the Related Art
A conventional lawn mower has freely rotatable dirigible front wheels, and rear drive wheels. A rear wheel differential mechanism is disposed on a drive transmission line from the engine to the rear wheels. When making a turn, the right and left rear drive wheels are driven at different speeds. Another type of lawn mower has dirigible front drive wheels, and rear drive wheels. A front wheel differential mechanism is disposed on a drive transmission line from the engine to the front wheels, while a rear wheel differential mechanism is disposed on a drive transmission line from the engine to the rear wheels. When making a turn, the right and left front and rear drive wheels are driven at different speeds through the two differential mechanisms.
A differential mechanism can absorb a difference in rotation between right and left wheels occurring in time of making a turn, and is thus used as an indispensable component of an automobile or the like. However, where drive is transmitted to drive wheels through a differential mechanism, a difference in rotation between the right and left drive wheels is caused also by loads applied from the ground to the drive wheels in certain surface conditions. As a result, the vehicle will lose its straight running performance. This problem is serious for a lawn mower that runs on lawns or meadows in bad ground conditions, compared with an automobile that runs on paved roads. Further, when a sudden decrease occurs in the load applied from a lawn to one of the rear drive wheels, that rear drive wheel suddenly slips and rotates fast, thereby marring the lawn.
Thus, a conventional tractor has, instead of using a differential mechanism, a one-way clutch mechanism in each of the drive transmission lines to right and left drive wheels. The drive wheels are switchable by action of the one-way clutch mechanisms. Such a tractor is know from British Patent Publication GB2335716 or U.S. Pat. No. 4,733,743, for example. The known tractor achieves a smooth turning performance, without a differential mechanism, by breaking drive transmission to a rear drive wheel rotating excessively as it moves along an outer turning track. However, even a slight difference in rotation between the right and left rear wheels causes operation of a clutch mechanism. This reduces durability of the clutch mechanisms, and noise produced by the clutch mechanisms is far from negligible.
Further, a rice planting machine must move with a high straight running performance and sharp turning performance in a paddy field in very difficult surface conditions. The rice planting machine employs a technique of providing a clutch in each of drive transmission lines to right and left rear drive wheels, for breaking drive transmission to one of the rear drive wheels moving along an inner track when the machine makes a turn. Such rice planting machines are disclosed in Japanese Patent Publications (Unexamined) Nos. 2001-88731 and 1997-220946, for example.
A different type of rice planting machine is known from Japanese Patent Publication (Unexamined) No. 2001-278102. This rice planting machine has a differential mechanism mounted on a drive transmission line to right and left rear drive wheels, and clutches for the rear drive wheels. In time of making a turn, the differential mechanism is locked, and drive transmission to one of the rear drive wheels moving along an inner track is broken.
A rice planting machine is a special vehicle that runs in a rice field. Its body is relatively simple, and has a space under a body frame. It is therefore relatively easy to provide a control mechanism such as clutches for the right and left rear wheels.
On the other hand, a lawn mower needs a large mower unit vertically movably suspended under a body frame. It is difficult to provide a control mechanism for switching drive wheels. Thus, no mechanism has been employed for operating clutches for right and left rear wheels in response to steering of dirigible wheels.